This invention relates to antimony trisulfide photoconductive devices and more particularly to photoconductive targets for vidicon-type camera tubes.
Antimony trisulfide is a through conductive bulk resistance type photoconductive material which has long been used in targets for vidicon-type camera tubes. The dark current in such a target constitutes the conduction or leakage of electrons from an electron beam scanned surface of the target through an antimony trisulfide photoconductive body to a signal plate, in the dark or in the absence of illumination striking the target (i.e. the non-illuminated signal current). This leakage or non-illuminated signal current is considered to be objectionably high (on the order of about 20 nanoamperes per cm.sup.2 of target area at target voltages of from 30-40 volts) and is the result of the characteristic bulk resistivity of antimony trisulfide of about 10.sup.11 - 10.sup.12 ohms-cm.
In order to retain a desired contrast level between target areas which are illuminated or exposed to radiant energy and those which are not, it is desirable to have a signal current contrast ratio significantly greater than that ordinarily associated with prior art antimony trisulfide bodies. The signal current contrast ratio consists of the output signal current associated with an illuminated target minus the non-illuminated signal current divided by the non-illuminated signal current. It is therefore particularly desirable to provide a substantial increase in the effective through resistance of the target which exceeds the characteristic resistance of the antimony trisulfide body alone, without also adversely affecting the quantum efficiency and sensitivity of the target in order to achieve the desired improvement in the contrast ratio.
One technique which has been used to reduce the dark current of a photoconductive target employing antimony trisulfide is to coat the photoconductive body with a homogeneous insulating or dielectric layer as disclosed in U.S. Pat. No. 3,136,909; however, such a target structure is not through-conducting and only provides a transitory capacitance type readout which requires complicated erase and discharge techniques for sensing and storing successive images. Such target structures permit only a single transitory readout for each stored image.
Other photoconductive bodies employed as targets for vidicon-type camera tubes include semiconductor photoconductive materials which, unlike antimony trisulfide, are known in the prior art to be capable of fabrication in contact with other materials of opposite electrical conductivity to form a blocking contact having characteristics similar to p-n junctions at the interface between the disimilar materials. Prior art attempts to fabricate similar blocking contacts with photoconductive targets including antimony trisulfide bodies have proven ineffective generally and have resulted in inferior signal current contrast ratios in comparison with antimony trisulfide targets without such additional structure. Moreover, in such structures, a mosaic arrangement of such blocking contacts, or p-n junctions, must be fabricated because of the low resistance of suitable semiconductor materials. Such "structured targets" require more complicated and considerably more costly manufacturing techniques as compared with those normally associated with antimony trisulfide targets.